FILE - In this Oct. 29, 2010 file photo, Deputy National Security Adviser for Homeland Security and Counterterrorism John Brennan briefs reporters at the White House in Washington. Brennan, now President Barack Obama's nominee to be CIA director, withdrew from consideration for the job in 2008 amid criticism over the agency's use of harsh interrogation techniques, like waterboarding, against terrorist suspects. This time, in 2013, he's making it clear he strongly opposes such practices. Former and current U.S. intelligence officials say Brennan wasn't so vocal a decade ago. (AP Photo/Charles Dharapak, File)

WASHINGTON (Reuters) - President Barack Obama's nominee to head the Central Intelligence Agency, John Brennan, is expected to face tough questioning about leaks of sensitive information and U.S. spy activities from waterboarding to the use of drones when he appears at a Senate confirmation hearing on Thursday.

In a written submission posted on the Senate Intelligence Committee's website on Wednesday, Brennan acknowledged for the first time that he had given voluntary interviews in connection with investigations into leaks that are being conducted by federal prosecutors in Baltimore and Washington.

Brennan said the investigations related to cyber warfare against Iran and a foiled bomb plot tied to al Qaeda's Yemen-based affiliate. Brennan said his lawyer had been told by prosecutors that he was "only a witness" in both investigations.

However, leaks are only one of the major issues about which intelligence committee members plan to question Brennan, a former CIA executive under President George W. Bush, who has become the steward of Obama's drone policies.

Congressional aides said much of the questioning is expected to focus on what Brennan knew about the Bush administration's use of harsh interrogation techniques on Islamic militant suspects captured and held, sometimes in secret CIA prisons, after the September 11, 2001 attacks on the United States.

Human rights activists and many U.S. politicians, including intelligence committee chairwoman Senator Dianne Feinstein and Republican Senator John McCain, have condemned some of the interrogation techniques as torture.

Democrats also are expected to question Brennan, the top White House adviser on counterterrorism and homeland security, on the Obama administration's use of armed drone aircraft to attack suspected al Qaeda militants and encampments in countries including Afghanistan, Pakistan and Yemen.

Despite the range of expected questions, Senate aides and political handicappers say they have not sensed a groundswell of opposition to Brennan's nomination.

Brennan first surfaced as an Obama CIA nominee in 2008 but he withdrew after human rights activists protested his equivocal public statements regarding the agency's use of "enhanced interrogation" techniques, including the simulated drowning practice known as "waterboarding."

This time human rights groups have raised questions about Brennan, but opposition has come nowhere close to the level mounted by conservative and pro-Israel groups against Chuck Hagel, Obama's nominee to replace Leon Panetta at the Pentagon.

WATERBOARDING QUESTIONS

Brennan's knowledge of waterboarding and other harsh interrogation techniques is likely to provoke intense committee questioning.

As Reuters reported earlier this month based on sources, classified CIA message traffic shows that Brennan had detailed, contemporaneous knowledge of the CIA's use of "enhanced interrogation techniques," including waterboarding.

After he temporarily left government service in 2005, Brennan publicly renounced waterboarding and other physically painful interrogation techniques. He and Obama administration officials have also said that Brennan had objected privately to fellow intelligence officers about the techniques' use.

Former Bush administration officials involved in the program say they do not recall such objections.

In his written submissions to the intelligence committee, Brennan declared that he had "significant concerns and personal objections to many elements of the EIT (enhanced interrogation technique)" program while it was under way.

"I voiced those objections privately with colleagues at the agency," Brennan continued, adding: "When I left the agency, I spoke publicly about those concerns."

He maintained that after being named White House counter-terrorism advisor, "I was put in a position to influence decisions related to EITS, such as how we handle interrogations, and I strongly support the president's ban on such techniques."

The CIA, which under Bush ran a network of secret prisons overseas where many of the so-called enhanced interrogations were conducted, was now "out of the detention business and it should stay that way."

DRONE PROCEDURES

Questions about drones are expected after the leak this week of an unclassified Justice Department paper outlining Obama administration legal justifications for using armed drones to attack U.S. citizens alleged to be involved in terrorist plots.

In his written submissions to the panel, Brennan said drone strikes were targeted "against specific al Qaeda terrorists in order to prevent terrorist attacks on the United States and to save American lives." He insisted those attacks were conducted "in full compliance with the law."

He said drone strikes hit their targets with "astonishing precision" and therefore "dramatically reduce the danger to U.S. personnel and innocent civilians."

LEAK INVESTIGATIONS

Brennan is also likely to face questions about the news leaks, which are under federal investigation, although a congressional source said no evidence has emerged yet that Brennan played a role in some of the leaks.

The U.S. attorney in Baltimore is examining leaks about the alleged role of U.S. agencies in cyber-warfare activities against Iran, including deployment of a virus known as Stuxnet.

The U.S. attorney in Washington is looking into leaks which led to the premature end of an undercover operation to infiltrate al Qaeda in the Arabian Peninsula (AQAP), the militant network's Yemen-based affiliate.

The Associated Press first reported last May that the U.S. had disrupted a plot to bomb an airliner with a newly designed "underwear bomb."

Brennan subsequently advised former U.S. counter-terrorism officials who are TV news pundits that the plot was never a serious threat because the U.S. had "inside control."

This led to a statement by one TV pundit that the United States apparently had "somebody on the inside" of the plot. Reuters later learned that British intelligence agencies helped to plant an informant in AQAP, and that their operation, in which Saudi and U.S. agencies also cooperated, had to be wound up prematurely because of leaks.